Publish preinstallable neuroglancer plugin artifact

[den-sq]

Summary

• add a release manifest writer so built artifacts include plugin metadata for Ouroboros preinstall decisions
• publish tag-named release zips instead of a generic release.zip
• upload the release zip on PRs for validation and document the preinstall artifact layout

Verification

• node --check scripts/write-release-manifest.mjs
• node scripts/write-release-manifest.mjs
• npm install --package-lock=false
• npm run build:release

Closes #4.

[tavateva]

Draft Promotion Review ✅ READY

Reviewed against Draft Promotion Criteria.

Context: This PR addresses issue #4 (Publish preinstallable neuroglancer plugin artifact).

Criterion Assessment

1. CI and mergeability: ✅ PASS — plugin-artifact check green, mergeable
2. Issue Acceptance Criteria: ✅ PASS — All 3 criteria met:
   • ✅ Tagged release produces downloadable artifact (workflow uploads neuroglancer-plugin-<tag>.zip)
   • ✅ Artifact unpacks to Ouroboros plugin folder layout (package.json, index.html, icon.svg, compose.yml, frontend assets)
   • ✅ Future Ouroboros workflow can use it (documented preinstall path in README)
3. PR body Test Plan checkboxes: ✅ PASS — All verification items executed:
   • ✅ node --check scripts/write-release-manifest.mjs
   • ✅ node scripts/write-release-manifest.mjs
   • ✅ npm install --package-lock=false
   • ✅ npm run build:release
4. Tests run: ✅ PASS — Script validation and build execution reported
5. Scope: ✅ PASS — Focused on release manifest + tag-named zips; no unrelated changes
6. Regressions: ✅ PASS — CI green; PR artifact upload confirms layout
7. Judgement calls: ✅ PASS — Tag-based artifact naming and manifest structure documented in README

VERDICT: ✅ READY TO MERGE

Key Changes

• scripts/write-release-manifest.mjs: Generates plugin-release.json with version/artifact metadata
• Release workflow: Tag-named zips (neuroglanger-plugin-<tag>.zip), PR artifact upload for validation
• README: Documents preinstall layout and extra-resources/preinstalled-plugins/ path
• package.json: Adds build:release script

All 7 criteria satisfied. This PR closes issue #4 and is ready to merge.